1. Field of the Invention
The invention relates generally to well safety systems and flow control systems for underground gas storage wells and reservoirs and the like. In particular aspects, the invention relates to well safety systems having a plurality of surface controlled subsurface safety valves.
2. Description of the Related Art
Storage wells are used to provide temporary underground storage for excess natural gas that has been produced. Storage wells are often located in underground salt domes, thereby providing a large underground storage cavern. In this instance, it is necessary to drill a wellbore that communicates with the surface. Gas is injected into the storage well and then removed at a later time when needed. To accomplish the injection of gas, a tubing string is typically disposed into the wellbore, suspended from a wellhead. Gas is injected down the annulus between the tubing string and the wellbore wall. Because water is typically present in the gas storage well, it must be removed to accommodate the injected gas. Valving within the wellhead tree controls the flow of gas and water into and out of the cavern, respectively.
An important design feature for a gas storage well is the ability to provide high fluid flow rates. In modern storage wells, high flow rates are needed during injection of gas as well as production of gas from the well. Some systems, for example, are required to provide production rates of 200 million to 300 million cubic feet per day of gas. Conventional flow control systems are generally unable to provide the high flow rates that are necessary.
A second important design feature for a gas storage well is the ability to close off the well below the surface quickly in the event of an emergency. Wellhead tree valves are not useful in the event of an emergency requiring that gas flow be shut off at a point well below the surface of the well. If there is no subsurface safety valve present, the well cannot be safely shut off in the event of an emergency. Inflatable packers are sometimes used as temporary barriers to close off portions of the well below the surface. However, even these are not useful in an emergency where rapid shut down is required. These barriers require downhole control in that tools must be run in to the wellbore to actuate them.
Some systems are known that have employed surface controlled subsurface valves to control flow into and/or out of a gas storage well. In general, however, these are complex tools of highly specialized construction and, as a result, quite costly. Additionally, these arrangements are generally unable to perform simultaneous injection of gas and leaching of water. Water flow out of the storage cavern is conducted through a portion of the flowbore of the tubing string and then laterally outwardly into the annulus. Therefore, injection of gas must be stopped to allow water to be leached. This is undesirable.
The present invention addresses the problems of the prior art.